1. Field of the Invention
The present invention relates fan operation verification and speed measurement techniques, and, more particularly, to methods and apparatus for sensing the rotation and/or speed of a brushless DC fan motor.
2. Description of the Related Art
Fans are standard components in many commercial computers, workstations, notebook and desktop personal computer systems, and in many other electronic and mechanical systems. Fans remove heat generated due to operation of integrated circuits and other components in such systems. An example of a common type of fan is the brushless DC fan. Brushless fans do not include a conductor serving to maintain an electrical connection between stationary and moving parts of the fan. Brushless DC fans have become standard components in many computer systems.
Inadequate cooling due to a fan failure in a system including electronic components may subject the system to a thermal runaway condition resulting in failure of one or more of the electronic components. Component failure often results in expensive repair or replacement of the components or of the entire system. Proper fan operation is therefore critical to the continued operation of such systems, and great importance is placed on early detection of impending fan failures.
The most common causes of fan failure are mechanical in nature. For example, abnormal motor bearings can cause a fan to run at a less than rated speed which in turn can increase the temperature within the system enclosure. Other causes of fan failure include open or shorted fan motors (e.g., no fan rotation due to an open or short circuit in the motor power supply lines, power connector or in the fan itself).
Fan failures of these and other types can be detected by comparing actual fan speed to other system parameters such as a measured system temperature and a fan speed control signal. If the measured fan speed is inconsistent with these parameters, abnormal fan operation is indicated. Many computer systems include fan fault detection techniques. One common fan fault detection technique uses a sensor located adjacent to the fan to monitor air flow. Another common fan fault detection scheme uses a custom fan assembly having a built-in rotational sensor that produces an output waveform proportional to fan speed.
However, the above mentioned fan fault detection techniques often require a custom-made fan which is typically more expensive and less commercially available than standard fans which are used in most personal computer systems. Also, the use of custom fans causes problems with component interchangeability. For example, although a typical fan that is mounted in a personal computer system receives direct current (DC) power from a two terminal interface connector located within the personal computer system, a custom fan assembly often requires a non-standard three terminal electrical connector.
A sensor for monitoring air flow includes additional disadvantages in that an air flow sensor must be mounted in a proper location with respect to the fan to properly sense the air flow, thereby complicating the mechanical layout and assembly of the system, increasing assembly cost and possibly compromising system reliability. Also, air flow sensors are often expensive.